Wheel bearing apparatus to support a vehicle wheel is an apparatus that rotatably supports a wheel mounting wheel hub, via double row rolling bearings, for a driving wheel and a driven wheel. In general, for structural reasons, a bearing of the inner ring rotation type is used for a driving wheel. Both inner ring rotation and outer ring rotation types are used for a driven wheel. In general, the wheel bearing apparatus is classified into a so-called first-generation type, where the wheel bearing includes double row angular ball bearings fitted between a knuckle and a wheel hub, a second-generation type, where the body mounting flange or the wheel mounting flange is directly formed on the outer circumferential surface of the outer member, a third generation type, where one of the inner raceway surfaces is formed directly on the outer circumferential surface of the wheel hub, and a fourth generation type, where an inner raceway surface is formed directly on the outer circumferential of an outer joint member of the wheel hub and a constant velocity universal joint, respectively.
In the bearing portion of these wheel bearing apparatus, seals are provided to prevent leakage of grease contained within the bearing as well as to prevent ingress of rain water or dust into the bearing. Recently, the automobile industry has trended toward maintenance-free bearings which require the bearings to have a longer life. From investigations into what causes damage to the bearings, it has been found that a substantial part of the damage is caused by troubles with the seal of bearing rather than a cause such as delamination of bearing surface. Accordingly, the life of the bearing apparatus can be improved by improving the sealability and durability of the bearing apparatus.
Various seals have been proposed for the wheel bearing apparatus to improved sealability. One representative example is shown in FIG. 17. This seal includes an annular sealing plate 53 and a sealing element 54. Each has an “L” shaped cross-section and is arranged opposite to one another. They are mounted on an inner member 51, such as an inner ring of the bearing, and an outer member 52, such as the outer ring of bearing in order to seal a space between the inner and outer members 51 and 52.
The sealing plate 53 includes a cylindrical portion 53a, press fit onto the outer circumferential surface of the inner member 51, a standing plate portion 53b, upstanding from the cylindrical portion 53a, and an inclined plate portion 53c, which extend from the tip of the standing plate portion 53b toward the inside of the seal. The sealing plate 53 functions as a slinger and is formed by press working of a steel sheet.
The sealing element 54 includes a steel base plate 55, with an “L” shaped cross-section and an elastomeric member 56, formed from rubber or synthetic resin. The sealing elements 54 is mounted on the outer member 52 by press fitting a cylindrical portion 55a of the base plate 55 onto the inner circumferential surface of the outer member 52. A standing plate portion 55b of the base plate 55 has an inclined portion 55c which extends from the tip of the standing plate portion 55b towards the inside of the seal. The elastomeric member 56 is arranged on the inside surface of the base plate 55 to cover it. The elastomeric member 56 has first and second side lips 56a and 56b and one radial lip 56c. 
The first side lip 56a extends from the standing plate portion 55b of the base plate 55 and has a radially outward inclination. The first side lip tip slidingly contacts the inclined plate portion 53c of the sealing plate 53. The second side lip 56b extends from a position near the tip of the standing plate portion 55b and has a radially outward inclination. The radial lip 56c extends from the tip of the standing plate portion 55b and has a radially inward inclination opposite to the standing plate portion 53b of the sealing plate 53. The radial lip tip slidingly contacts the cylindrical portion 53a. 
Muddy water or dust splashed on the seal is directed onto the outer surface of the first side lip 56a which slidably contacts the inner surface of the sealing plate 53. However, due to the inclined plate portion 53c, extending radially outward from the standing plate portion 53b of the sealing plate 53, a small space “S” is formed into which muddy water or dust can enter. Thus, reduction of the sealing performance of the first side lip 56a can be caused by the accumulation of foreign matter, such as mud or sands, contained in the muddy water.
If the inner member 51 is arranged at a rotational side, muddy water or foreign matter contained in the space “S” will be splashed out by centrifugal force. Thus, it is possible to obtain an action of rapid discharge of the foreign matter without reducing the sealing performance. Even though the muddy water or foreign matter would immigrate into the next space between the first and second side lips 56a and 56b, further immigration of the foreign matter, etc. will be prevented by the second side lip 56b. Furthermore, if the foreign matter etc. immigrates into the next adjacent space between the second side lip 56b and the radial lip 56c, further immigration of the foreign matter etc. into the bearing will be prevented by the radial lip 56c. The radial lip 56c also can prevent leakage of lubricating oil contained within the bearing. See, Reference Patent Document: Japanese Laid-open Patent Publication No. 292032/1997
Although such a prior art seal can perform triple action sealing by the first and second side lips 56a and 56b as well as the radial lip 56c to prevent immigration of muddy water or other foreign matters and obtain a high sealing performance, rotational torque is increased by the friction resistance caused by the triple seals 56a, 56b and 56c. Thus, fuel consumption will be increased. In addition, heat generation in the seal will reduce the durability of the seal and will cause adverse influence on the life of bearing.